Barcodes are standardized patterns of marks and spaces that are widely used for identification of goods and persons, among other things. The marks and spaces of the barcodes are in the form of indicia of variable reflectivity, some being light while others are dark. So-called "quiet zones" of high-reflectivity lead and trail the marks and spaces that serve to initialize and terminate the barcodes.
To read the barcodes, light is irradiated on a reading window where the barcodes are to be present, such as the channel through which a bar-coded badge is swiped, and as each bar code is moved through the reading window light reflected off the quiet-zones and off the marks and spaces of variable reflectivity is deviated to a light detector. The electrical pulses of the detector correspond in pattern and intensity to the incident light. The electrical pulses are typically amplified in an amplifier, squared-up in a comparator with hysteresis and digitally decoded.
The heretofore known barcode optical assemblies that implemented the functions of irradiating the reading window and deviating the reflected light to the photo detector typically included an aperture stop of size less than the minimum width of the marks and spaces and a folding mirror to deviate the reflected light onto a large-area photo detector. However, the utility of the heretofore known optical assemblies has been limited by the fact that the elements of the optical train including the aperture stop and folding mirror were difficult to align and to maintain in alignment, which resulted in undesirable time and labor costs during manufacture, and from the need to provide various separate metallic "shields" about the elements of the optical assembly in order to prevent noise from disrupting the operation of the photo detector, which often resulted in expensive as well as ungainly designs.
The heretofore known barcode reader circuits that implemented the amplification and pulse squaring-up functions typically included a high-gain amplifier connected to a comparator configured with hysteresis. The utility of the heretofore known barcode reader circuits was limited, however, by the fact that manual adjustment of potentiometers was required to bring the voltage swings of the photodiode output pulses to within the dynamic range of the amplifier, which resulted in undesirable time and labor costs during manufacture, from the fact that the amplifier typically was driven beyond saturation to eliminate from the photo detector pulses peak noise, which often resulted in erroneous output transitions, (disrupting the decoding of the message), and from the fact that the comparator configured with hysteresis tended to "latch" in the absence of signal, which resulted in error in downstream digital signal processing.